1. Field of the Invention
The present invention relates, generally, to cell-type filter cartridges and more particularly to filter cartridges having a plurality of cells with netting or mesh disposed on the outside of the filter material.
2. Description of the Prior Art
Cell-type filter cartridges have long been used and are being increasingly used in a variety of situations. These type of filter cartridges are typically fabricated from individual cells generally having two layers of filter media separated from each other. Liquid would typically flow from the outside of the filter medium into the space between the layers of the filter media towards the central portion of the cell. A significant advantage to cell-type filter cartridges is that the surface area of filter material is quite large when compared to the total volume of an assembled cell-type filter cartridge.
Disposed between each filter medium is a separator which is typically formed in the shape of disks having ribs radially extending from the central aperture in a spoke-like pattern. In addition to separating the two layers of filter media, it provides for fluid flow from the filter media towards the central aperture of the filter media. An excellent example of a filter separator may be found in U.S. Pat. No. 4,783,262 to Ostreicher et al., entitled "Improved Separator for Cell Type Filter Cartridges", assigned to the same applicant as the present invention and which is specifically incorporated herein by reference. The separator disk described therein has stiffening members formed at the central aperture of the cell which are attached to a plurality of separating ribs to thereby provide a rigid, box-like structure sufficient to impart substantial cantilever strength to the ribs. Further, one of the stiffening disks is positioned proximate the ends of the separator ribs in order to act as a loadbearing surface to prevent media intrusion of the filtering medium and to prevent blocking the area of the flow path with the filtered liquid. Accordingly, one distinct advantage in this type of separator is that during backwashing or reverse flow (i.e. fluid flow from the central aperture of the filter cell out towards the surface of the filter medium) damage to the filter media is minimized or ameliorated.
Filter cell cartridges use a variety of filter media for filtering many fluids. Examples of cellulosic fibrous filter media and uses may be found, for example, in U.S. Pat. Nos. 4,617,128 "Particulate Filter Aid, Filter Bed and Process" dated Oct. 14, 1986, to Ostreicher; 4,309,247 "Filter and Method of Making Same" dated Jan. 5, 1982 to Hou et al.; 4,305,782 "Filter and Method of Making Same" dated Dec. 15, 1981 to Ostreicher et al.; 4,007,113 "Particulate Filter Medium and Process" dated Feb. 8, 1977 to Ostreicher; and 4,007,114 "Fibrous Filter Medium and Process" dated Feb. 8, 1977 to Ostreicher. All of the foregoing patents are incorporated herein by reference.
Use of a cell-type filter cartridge may be also found in U.S. Pat. No. 4,361,486 "Filter Media, Method for Oxidizing and Removing Soluble Iron, Method for Removing Inactivating Microorganisms and Particulate Filter Aid" issued Nov. 30, 1982 to Hou et al. which is incorporated herein by reference.
A process for manufacturing filter cells is described in U.S. Pat. No. 4,347,208 "Method of Making Filter Cell Having Sealed Periphery" issued Aug. 31, 1982 to Southall. In this patent, a filter cell cartridge is described which comprises a plurality of filter cells. Each of the filter cells is comprised of filter media having a conical separator therebetween with the periphery or edges of the filter cell being held together and sealed by an injection-molded flange.
Generally, all of the aforedescribed filter cartridges perform satisfactorily for their intended purpose during normal operating conditions. However, due to the inherent labor and capital costs in replacing the filter cartridge, most applications utilize a backwash or reverse flow process in order to improve volumetric fluid flow therethrough and prolong the life of the cartridge. Obviously, the more backwashing operations that are permissible, the lower the total operating costs of the system. However, during backwashing operations, many filters, including filter cell-type cartridges, experience decreased tensile strength of the media, cracking of the edge seal, masking off of the media and/or flaking off of actual filter media fibers. An illustration of this process is shown in FIG. 14 (prior art) wherein fluid, designated A, backflowing through filter media B causes shredding or flaking of the cell medium and the production of frayed edges of particles C. Another problem with filter-cell type cartridges is that unintentional backpressure applied to the filter cell may cause cell rupture and/or distortion.
A number of attempts have been made throughout the years to strengthen filters and to control the flow of fluid therethrough by the use of lattice-type structures external to the actual filter medium. An early example of this may be found in U.S. Pat. No. 445,223 entitled "Filter" issued Jan. 27, 1891 to E. M. Knight. The Knight patent utilizes a wire screen external to the filter media in order to prevent sagging of the filter media during use, the filter being partially held by the screen. The Knight patent found this feature important since the filter itself used a charcoal paste adjacent a fibrous covering and was hence "limp".
A second example may be found in U.S. Pat. No. 2,249,063 entitled "Filter Leaf" issued July 15, 1941 to Swem. This patent utilizes a wire screen to provide structural support to the filter media under the high operating temperature and pressure utilized.
Another example may be found in U.S. Pat. No. 2,263,853 entitled "Filter" issued Nov. 25, 1941 to Re Qua. This patent utilizes a metal ribbon twisted about its longitudinal axis in order to support the filter media. Additionally, a supporting screen was utilized which is simultaneously welded at diametrically opposed points on the ribbon.
In U.S. Pat. No. 4,274,964 "Dialysator" issued June 23, 1981 to Krick et al. A membrane-type hose in conjunction with an intermediate netting layer is used in an interlocking manner for the purpose of providing an improved flow pattern.
Accordingly, the above-mentioned patents use a support of one kind or another for the express purpose of providing a stiff filter support for the "limp" filter during use, i.e. the supports in effect form for filter elements which, by themselves, lack the appropriate physical properties. As such, the prior art braces or nets are used with filters specifically for the purpose of supporting the filter during forward fluid flow conditions as distinct from dealing with the problem of masking off, flaking off of actual filter media fibers, cracking at the edge seal and distortion caused by backwashing and backflow conditions and other related problems. In effect, the problem to be solved by the present invention is not one of physical integrity during normal operation and use, but rather the reverse of maintaining physical integrity where fluid is flowing contrary to normal flow as during backwashing operations or unintentional backpressure.